Energy conversion, the transformation of energy from forms provided by nature to forms that can be used by humans. Over the centuries a wide array of devices and systems has been developed for this purpose.

Some of these energy converters are quite simple.

The early windmills, for example, transformed the kinetic energy of wind into mechanical energy for pumping water and grinding grain This article traces the development of energy-conversion technology, appearing in a poem by an early Greek writer celebrating the liberation from toil of the young The first analysis of the performance of waterwheels was published in 1759 by College of Earth and Mineral Sciences - Efficiency of Energy Conversion..

Other energy-conversion systems are decidedly more complex, particularly those that take raw energy from fossil fuels and nuclear fuels to generate electrical power. Systems of this kind require multiple steps or processes in which energy undergoes a whole series of transformations through various intermediate forms.

Many of the energy converters widely used today involve the transformation of thermal energy into electrical energy. The efficiency of such systems is, however, subject to fundamental limitations, as dictated by the laws of thermodynamics and other scientific principles.

In recent years, considerable attention has been devoted to certain direct energy-conversion devices, notably solar cells and fuel cells, that bypass the intermediate step of conversion to heat energy in electrical power generation. This article traces the development of energy-conversion technology, highlighting not only conventional systems but also alternative and experimental converters with considerable potential.

It delineates their distinctive features, basic principles of operation, major types, and key applications. For a discussion of the laws of thermodynamics and their impact on system design and performance, General considerationsEnergy is usually and most simply defined as the equivalent of or capacity for doing work.

The word itself is derived from the Greek energeia: en, “in”; ergon, “work. ” Energy can either be associated with a material body, as in a coiled spring or a moving object, or it can be independent of matter, as light and other electromagnetic radiationtraversing a vacuum.

The energy in a system may be only partly available for use. The dimensions of energy are those of work, which, in classical mechanics, is defined formally as the product of mass (m) and the square of the ratio of length (l) to time (h1; thus the work done on the ball by the force of gravity acting on it in this cycle of events is zero.

Varying degrees of conversion in real systemsAlthough the total amount of energy in an isolated system remains unchanged, there may be a great difference in the quality of different forms of energy.

Many forms of energy, in theory, can be transformed completely into work or into other forms of energy. This is true for mechanical energy and electrical energy.

The random motions of constituent parts of a material associated with thermal energy, however, represent energy that is not available completely for conversion into directed energy. The French engineer Sadi Carnot described (in 1824) a theoretical power cycle of maximum efficiency for converting thermal into mechanical energy.

He demonstrated that this efficiency is determined by the magnitude of the temperatures at which heat energy is added and waste heat is given off during the cycle. A practical engine operating on the Carnot cycle has never been devised, but the Carnot cycle determines the maximum efficiency of thermal energy conversion into any form of directed energy.

The Carnot criterion renders 100 percent efficiency impossible for all heat engines. In effect, it constitutes the basis for what is now the second law of L.

RussellThe Editors of Encyclopaedia BritannicaHistory of energy-conversion technologyEarly attempts to harness natural forms of energyEarly humans first made controlled use of an external, nonanimal energy source when they discovered how to use fire.

Burning dried plant matter (primarily wood) and animal waste, they employed the energy from this biomass for heating and cooking. The generation of mechanical energy to supplant human or animal power came very much later—only about 2,000 years ago—with the development of simple devices to harness the energy of flowing water and of wind.

The earliest machines were waterwheels, first used for grinding grain. They were subsequently adopted to drive sawmills and pumps, to provide the bellows action for furnaces and forges, to drive tilt hammers or trip-hammers for forging iron, and to provide direct mechanical power for textile mills.

Until the development of steam power during the Industrial Revolution at the end of the 18th century, waterwheels were the primary means of mechanical power production, rivaled only occasionally by windmills. Thus, many industrial towns, especially in early America, sprang up at locations where water flow could be assured all year.

Before the Industrial Revolution, power came from three main sources: humans, draft animals, and ingenuity people used in harnessing waterpower can be seen in this medieval-style mill.

The waterwheel is turned by a stream and is connected to a shaft that leads into the building 0.2 The New Energy Conversion Technologies programme. 2. 0.3 Exergy. 4 In the case of the coal-fired steam boiler, a gas turbine cycle with an external heat..

The connection of a series of gears translates the power from the stream to a shaft that drives a millstone, which grinds flour from grain.

84MB)Public DomainThe oldest reference to a water mill dates to about 85 bce, appearing in a poem by an early Greek writer celebrating the liberation from toil of the young women who operated the querns (primitive hand mills) for grinding corn.

According to the Greek geographer Strabo, King Mithradates VI of Pontus in Asia used a hydraulic machine, presumably a water mill, by about 65 bce. Early vertical-shaft water mills drove querns where the wheel, containing radial vanes or paddles and rotating in a horizontal plane, could be lowered into the stream.

The vertical shaft was connected through a hole in the stationary grindstone to the upper, or rotating, stone. The device spread rapidly from Greece to other parts of the world, because it was easy to build and maintain and could operate in any fast-flowing stream.

It was known in China by the 1st century ce, was used throughout Europe by the end of the 3rd century, and had reached Japan by the year 610. Users learned early that performance could be improved with a millrace and a chute that would direct the water to one side of the wheel.

A horizontal-shaft water mill was first described by the Roman architect and engineer Vitruvius about 27 bce. It consisted of an undershot waterwheel in which water enters below the centre of the wheel and is guided by a millrace and chute.

The waterwheel was coupled with a right-angle gear drive to a vertical-shaft grinding wheel. This type of mill became popular throughout the Roman Empire, notably in Gaul, after the advent of Christianity led to the freeing of slaves and the resultant need for an alternative source of power.

8 metres (6 feet) in diameter, are estimated to have produced about three horsepower, the largest amount of power produced by any machine of the time.

The Roman mills were adopted throughout much of medieval Europe, and waterwheels of increasing size, made almost entirely of wood, were built until the 18th century. In addition to flowing stream water, ocean tides were used to drive waterwheels.

Tidal water was allowed to flow into large millponds, controlled initially through lock-type gates and later through flap valves. Once the tide ebbed, water was let out through sluice gates and directed onto the wheel.

Sometimes the tidal flow was assisted by building a dam across the estuary of a small river. Although limited in operation to ebbing tide conditions, tidal mills were widely used by the 12th century.

The earliest recorded reference to tidal mills is found in the Domesday Book (1086), which also records more than 5,000 water mills in England south of the Severn and Trent rivers. (Tidal mills also were built along the Atlantic coast in Europe and centuries later on the eastern seaboard of the United States and in Guyana, where they powered sugarcane-crushing mills.

)The first analysis of the performance of waterwheels was published in 1759 by John Smeaton, an English engineer. Smeaton built a test apparatus with a small wheel (its diameter was only 0.

61 metre) to measure the effects of water velocity, as well as head and wheel speed. He found that the maximum efficiency (work produced divided by potential energy in the water) he could obtain was 22 percent for an undershot wheel and 63 percent for an overshot wheel (i.

, one in which water enters the wheel above its centre).

In 1776 Smeaton became the first to use a cast-iron wheel, and two years later he introduced cast-iron gearing, thereby bringing to an end the all-wood construction that had prevailed since Roman times.

Based on his model tests, Smeaton built an undershot wheel for the London Bridge waterworks that measured 4 When all forms of energy coming out of an energy conversion device are In the case of an electric bulb, the electrical energy is converted to light and heat..

The results of Smeaton’s experimental work came to be widely used throughout Europe for designing new wheels. During the mid-1700s a reaction waterwheel for generating small amounts of power became popular in the rural areas of England.

In this type of device, commonly known as a Barker’s mill, water flowed into a rotating vertical tube before being discharged through nozzles at the end of two horizontal arms. These directed the water out tangentially, much in the way that a modern rotary lawn sprinkler does.

A rope or belt wound around the vertical tube provided the power takeoff. Early in the 19th century Jean-Victor Poncelet, a French mathematician and engineer, designed curved paddles for undershot wheels to allow the water to enter smoothly.

His design was based on the idea that water would run up the surface of the curved vanes, come to rest at the inner diameter, and then fall away with practically no velocity. This design increased the efficiency of undershot wheels to 65 percent.

At about the same time, William Fairbairn, a Scottish engineer, showed that breast wheels (i. , those in which water enters at the 10- or two-o’clock position) were more efficient than overshot wheels and less vulnerable to flood damage. He used curved buckets and provided a close-fitting masonry wall to keep the water from flowing out sideways.

In 1828 Fairbairn introduced ventilated buckets in which gaps at the bottom of each bucket allowed trapped air to escape. Other improvements included a governor to control the sluice gates and spur gearing for the power takeoff.

During the course of the 19th century, waterwheels were slowly supplanted by water turbines. Water turbines were more efficient; design improvements eventually made it possible to regulate the speed of the turbines and to run them fast enough to drive electric generators.

This fact notwithstanding, waterwheels gave way slowly, and it was not until the early 20th century that they became largely obsolescent. Yet even today some waterwheels still survive; in the early 1970s there were more than 1,000 grain mills in use in Portugal alone.

Equipped with submerged bearings, these modern waterwheels certainly are more sophisticated than their predecessors, though they bear a remarkable likeness to them. Windmills, like waterwheels, were among the original prime movers that replaced animal muscle as a source of power.

They were used for centuries in various parts of the world, converting the energy of the wind into mechanical energy for grinding grain, pumping water, and draining lowland areas. The first known wind device was described by Hero of Alexandria (c.

It was modeled on a water-driven paddle wheel and was used to drive a piston pump that forced air through a wind organ to produce sound.

The earliest known references to wind-driven grain mills, found in Arabic writings of the 9th century, refer to a Persian millwright of 644 ce, although windmills may actually have been used earlier. These mills, erected near what is now the Iran–Afghanistan border, had a vertical shaft with paddlelike sails radiating outward and were located in a building with diametrically opposed openings for the inlet and outlet of the wind.

Each mill drove a single set of stones without gearing. The first mills were built with the millstones above the sails, patterned after the early waterwheels from which they were derived.

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Adopting the ideas from contemporary waterwheels, builders began to use fabric-covered, wood-framed sails located above the millstone, instead of a waterwheel below, to drive the grindstone through a set of gears. The whole mill with all its machinery was supported on a fixed post so that it could be rotated and faced into the wind.

The millworks were initially covered by a boxlike wooden frame structure and later often by a “round-house,” which also provided storage. A brake wheel on the shaft allowed the mill to be stopped by a rim brake.

A heavy lever then had to be raised to release the brake, an early example of a fail-safe device. Mills of this sort first appeared in France in 1180, in areas of Syria under the control of the crusaders in 1190, and in England in 1191.

The earliest known illustration is from the Windmill Psalter made in Canterbury, England, in the second half of the 13th century. The large effort required to turn a post-mill into the wind probably was responsible for the development of the so-called tower mill in France by the early 14th century.

Here, the millstone and the gearing were placed in a massive fixed tower, often circular in section and built of stone or brick. Only an upper cap, normally made of wood and bearing the sails on its shaft, had to be rotated.

Such improved mills spread rapidly throughout Europe and later became popular with early American settlers. The Low Countries of Europe, which had no suitable streams for waterpower, saw the greatest development of windmills.

Dutch hollow post-mills, invented in the early 15th century, used a two-step gear drive for drainage pumps. An upright shaft that had gears on the top and bottom passed through the hollow post to drive a paddle-wheel-like scoop to raise water.

The first wind-driven sawmill, built in 1592 in the Netherlands by Cornelis Cornelisz, was mounted on a raft to permit easy turning into the wind. At first both post-mills and the caps of tower mills were turned manually into the wind.

Later small posts were placed around the mill to allow winching of the mill with a chain. Eventually winches were placed into the caps of tower mills, engaged with geared racks and operated from inside or from the ground by a chain passing over a wheel.

Tower mills had their sail-supporting or tail pole normally inclined at between 5° and 15° to the horizontal. This aided the distribution of the huge sail weight on the tail bearing and also provided greater clearance between the sails and the support structure.

Windmills became progressively larger, with sails from about 17 to 24 metres in diameter already common in the 16th century. The material of construction, including all gearing, was wood, although eventually brass or gunmetal came into use for the main bearings.

Cast-iron drives were first introduced in 1754 by John Smeaton, the aforementioned English engineer. Little is known about the actual power produced by these mills.

In all likelihood only from 10 to 15 horsepower was developed at the grinding wheels. A 50-horsepower mill was not built until the 19th century.

The maximum efficiency of large Dutch mills is estimated to have been about 20 percent. In 1745 Edmund Lee of England invented the fantail, a ring of five to eight vanes mounted behind the sails at right angles to them.

These were connected by gears to wheels running on a track around the cap of the mill. As the wind changed direction, it struck the sides of the fantail vanes, realigning them and thereby turning the main sails again squarely into the wind.

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A major problem with all windmills was the need to feather the sails or reduce sail area so that if the wind suddenly increased during a storm the sails would not be ripped apart. In 1772 Andrew Meikle, a Scottish millwright, invented the spring sail, a shutter arrangement similar to a venetian blind in which the sails were controlled by a spring.

When the wind pressure exceeded a preset amount, the shutters opened to let some of the wind pass through. In 1789 Stephen Hooper of England introduced roller blinds that could all be simultaneously adjusted with a manual chain from the ground while the mill was working.

This was improved upon in 1807 by Sir William Cubitt, who combined Meikle’s shutters with Hooper’s remote control by hanging varying weights on the adjustment chain, thus making the control automatic. These so-called patent sails, however, found acceptance only in England and northern Europe.

Even though further improvements were made, especially in speed control, the importance of windmills as a major power producer began to decline after 1784, when the first flour mill in England successfully substituted a steam engine for wind power. Yet, the demise of windmills was slow; at one time in the 19th century there were as many as 900 corn (maize) and industrial windmills in the Zaan district of the Netherlands, the highest concentration known.

Windmills persisted throughout the 19th century in newly settled or less-industrialized areas, such as the central and western United States, Canada, Australia, and New Zealand. They also were built by the hundreds in the West Indies to crush sugarcane.

The primary exception to the steady abandonment of windmills was resurgence in their use in rural areas for pumping water from wells. The first wind pump was introduced in the United States by David Hallay in 1854.

After another American, Stewart Perry, began constructing wind pumps made of steel and equipped with metal vanes in 1883, this new and simple device spread around the world. Wind-driven pumps remain important today in many rural parts of the world.

They continued to be used in large numbers, even in the United States, well into the 20th century until low-cost electric power became readily available in rural areas. Although rather inefficient, they are rugged and reliable, need little attention, and remain a prime source for pumping small amounts of water wherever electricity is not economically WailesFred LandisDevelopments of the Industrial RevolutionThe rapid growth of industry in Britain from about the mid-18th century (and somewhat later in various other countries) created a need for new sources of motive power, particularly those independent of geographic location and weather conditions.

This situation, together with certain other factors, set the stage for the development and widespread use of the steam engine, the first practical device for converting thermal energy to mechanical energy. The foundations for the use of steam power are often traced to the experimental work of the French physicist Denis Papin.

In 1679 Papin invented a type of pressure cooker, a closed vessel with a tightly fitting lid that confined steam until high pressure was generated. Observing that the steam in the vessel raised the lid, he conceived the idea of using steam to power a piston and cylinder engine.

Thomas Savery, an English inventor and military engineer, studied Papin’s work and built a steam-driven suction machine for removing water from coal mines. Savery’s machine (patented in 1698) consisted of a boiler, a closed, water-filled reservoir, and a series of valves.

Steam was introduced into the reservoir, and the pressure of the steam forced the water out through a one-way outlet valve until the vessel was empty. Water was then sprayed over the surface of the vessel to condense the steam and create a vacuum capable of drawing up more water through a valve below.

Unfortunately the vacuum created was not perfect, and so water could only be lifted to a limited height. Some years later another English engineer, Thomas Newcomen, developed a more efficient steam pump consisting of a cylinder fitted with a piston—a design inspired by Papin’s aforementioned idea.

When the cylinder was filled with steam, a counterweighted pump plunger moved the piston to the extreme upper end of the stroke. With the admission of cooling water, the steam condensed, creating a vacuum.

The atmospheric pressure in the mine acted on the piston and caused it to move down in the cylinder, and the pump plunger was lifted by the resulting force. Because Savery had obtained a broad patent for his steam device, Newcomen could not patent his engine.

He thus entered into a partnership with Savery, and together they built, in 1712, the first piston-operated steam pump ENERGY CONVERSION TECHNOLOGY; HELP ME DO COLLEGE ENERGY Who can help me with my custom energy conversion technology case study In your own words write a definition for the term energy conversion; Who can help .

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Although engines of this kind converted only about 1 percent of the thermal energy in the steam to mechanical energy, they remained unrivaled for more than 50 years. Watt’s engineIn 1765 James Watt, a Scottish instrument maker and inventor, modified a Newcomen engine by adding a separate condenser to make it unnecessary to heat and cool the cylinder with each stroke.

Because the cylinder and piston remained at steam temperature while the engine was operating, fuel costs dropped by about 75 percent. Watt entered into a partnership with Matthew Boulton, who owned a factory in Soho, near Birmingham, England.

At Boulton’s insistence he set out to develop a new kind of engine that rotated a shaft instead of providing simple up-and-down motion. He found a way to obtain an inflexible connection between piston and rod (beam) and invented special gear arrangements to convert the up-and-down movement of the beam into circular motion.

A heavy flywheel was added to smooth out the variations in the force delivered to the engine shaft by the action of the piston in the cylinder. The flow of steam to the engine was regulated by a governor connected to the flywheel.

In addition, Watt applied steam to both sides of the piston to produce greater uniformity of effort and increased power. Although far more difficult to build, Watt’s rotative engine opened up an entirely new field of application: it enabled the steam engine to be used to operate rotary machines in factories and cotton mills.

The rotative engine was widely adopted; it is estimated that by 1800 Watt and Boulton had built 500 engines, of which less than 40 percent were pumps and the rest were of the rotative type. High-pressure steam enginesAlthough Watt understood the advantages of utilizing the expansive power of steam within a cylinder, he refused to use steam under high pressure for reasons of safety.

By the early years of the 19th century, however, the American inventor Oliver Evans had built a stationary high-pressure steam engine for driving a rotary crusher to produce pulverized limestone for agricultural use.

Within a few years Evans had designed lighter-weight high-pressure steam engines that could do various other tasks, such as drive sawmills, sow grain, and power a dredge. From 1806 to about 1816 he produced more than 100 steam engines that were employed with screw presses for processing paper, cotton, and tobacco.

Other major advances in the use of high-pressure steam were achieved by Richard Trevithick in England during the early years of the 19th century. Trevithick built the world’s first steam-powered railway locomotive in 1803.

Two years later he adapted his high-pressure steam engine to drive an iron-rolling mill and to propel a barge with the help of paddle wheels. Watt’s engine was able to convert only a little more than 2 percent of the thermal energy in steam to work.

The improvements introduced by Evans, Trevithick, and others (e. , three separate expansion cycles and higher steam temperatures) increased the efficiency of the steam engine to roughly 17 percent by 1900. Yet, within the next decade the steam engine was supplanted for various important applications by the more efficient steam turbine.

Owing to technological advances and the use of high-temperature steam, steam turbines have attained an efficiency of thermal energy conversion of approximately 40 t B. WoodruffThe Editors of Encyclopaedia BritannicaMany of the early high-pressure steam boilers exploded because of poor materials and faulty methods of construction.

The resultant casualties and property losses motivated Robert Stirling of Scotland to invent a power cycle that operated without a high-pressure boiler. In his engine (patented in 1816), air was heated by external combustion through a heat exchanger and then was displaced, compressed, and expanded by two pistons.

Stirling also conceived the idea of a regenerator to store thermal energy during part of the cycle and then return this energy to the working fluid.

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Nevertheless, until about 1920, small engines of this type were used to pump water on farms and to generate electricity for small communities.

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Modern versions of the Stirling engine employ pressurized hydrogen or helium instead of air. Since the 1970s the engine has been adapted for many uses, including cryogenic refrigeration, submarine propulsion, and electrical production.

While the steam engine remained dominant in industry and transportation during much of the 19th century, engineers and scientists began developing other sources and converters of energy. One of the most important of these was the internal-combustion engine.

In such a device a fuel and oxidizer are burned within the engine and the products of combustion act directly on piston or rotor surfaces. By contrast, an external-combustion device, such as the steam engine, employs a secondary working fluid that is interposed between the combustion chamber and power-producing elements.

By the early 1900s the internal-combustion engine had replaced the steam engine as the most broadly applied power-generating system not only because of its higher thermal efficiency (there is no transfer of heat from combustion gases to a secondary working fluid that results in losses in efficiency) but also because it provided a low-weight, reasonably compact, self-contained power plant. The German engineer Nikolaus August Otto is generally credited with having built the first practical internal-combustion engine (1876), though several rudimentary devices had appeared earlier in the century.

In 1885 Gottlieb Daimler, another German engineer, modified the four-cycle Otto engine so that it burned gasoline (instead of coal powder) and built the first successful high-speed internal-combustion engine. Within several decades the gasoline engine found wide application in motorcycles, automobiles, and small trucks.

Another type of internal-combustion engine was introduced by Rudolf Diesel, also of Germany, in the early 1890s. Named for its inventor, the diesel engine was more efficient than engines of the Otto variety and was fueled by heavy oil, which is cheaper and less volatile than gasoline.

As a result, it was adopted as the primary power plant for submarines, railway locomotives, and heavy machinery. An internal-combustion engine quite different from the reciprocating piston type was developed around the turn of the century.

This was the gas-turbine engine, the first successful version of which was built in 1903 in France. Modern gas turbines have been used for electric power generation and various other purposes, but its primary application has been jet propulsion.

In a gas-turbine system compressed air, heated by the combustion of petroleum, is used to turn a turbine to drive the compressor while excess energy accelerates the exhaust gas to high velocity for producing thrust. Another form of propulsive engine, the rocket, attracted increasing attention during the final decades of the 19th century due in part to the imaginative portrayals of space travel fabricated by Jules Verne and other science-fiction writers.

From about 1880, various scientists and inventors began investigating theoretical problems of rocket motion and propulsion system design. Goddard of the United States had developed experimental rockets employing liquid and solid propellants. Other important energy-conversion devices emerged during the 19th century.

During the early 1830s the English physicist and chemist Michael Faraday discovered a means by which to convert mechanical energy into electricity on a large scale. While engaged in experimental work on magnetism, Faraday found that moving a permanent magnet into and out of a coil of wire induced an electric current in the wire.

This process, called electromagnetic induction, provided the working principle for electric generators. During the late 1860s Z nobe-Th ophile Gramme, a French engineer and inventor, built a continuous-current generator.

Dubbed the Gramme dynamo, this device contributed much to the general acceptance of electric power. By the early 1870s Gramme had developed several other dynamos, one of which was reversible and could be used as an electric motor.

Electric motors, which convert electrical energy to mechanical energy, run virtually every kind of machine that uses electricity. All of Gramme’s machines were direct-current (DC) devices.

It was not until 1888 that Nikola Tesla, a Serbian-American inventor, introduced the prototype of the present-day alternating-current (AC) motor.

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In recent years, direct energy-conversion devices have received much attention because of the necessity to develop more efficient ways of transforming available forms of primary energy into electric power.

Four such devices—the electric battery, the fuel cell, the thermoelectric generator (or at least its working principle), and the solar cell—had their origins in the early 1800s The course covers topics such as photovoltaic, wind, thermo-mechanical energy conversion systems, hybrid renewable energy systems, energy efficiency, .

The battery, invented by the Italian physicist Alessandro Volta about 1800, changes chemical energy directly into an electric current. A device of this type has two electrodes, each of which is made of a different chemical.

As chemical reactions occur, electrons are released on the negative electrode and made to flow through an external circuit to the positive electrode. The process continues until the circuit is interrupted or one of the reactants is exhausted.

The forerunners of the modern dry cell and the lead-acid storage battery appeared during the second half of the 19th century. The fuel cell, another electrochemical producer of electricity, was developed by William Robert Grove, a British physicist, in 1839.

In a fuel cell, continuous operation is achieved by feeding fuel (e. , hydrogen) and an oxidizer (oxygen) to the cell and removing the reaction products. Thermoelectric generators are devices that convert heat directly into electricity.

Electric current is generated when electrons are driven by thermal energy across a potential difference at the junction of two conductors made of dissimilar materials. This effect was discovered by Thomas Johann Seebeck, a German physicist, in 1821.

Seebeck observed that a compass needle near a circuit made of different conducting materials was deflected when one of the junctions was heated. He investigated various materials that produce electric energy with an efficiency of 3 percent.

This efficiency was comparable to that of the steam engines of the day. Yet, the significance of the discovery of the thermoelectric effect went unrecognized as a means of producing electricity because of Seebeck’s misinterpretation of the phenomenon as a magnetic effect caused by a difference in temperature.

A basic theory of thermoelectricity was finally formulated during the early 1900s, though no functional generators were developed until much later. In a solar cell, radiant energy drives electrons across a potential difference at a semiconductor junction in which the concentrations of impurities are different on the two sides of the junction.

What is often considered the first genuine solar cell was built in the late 1800s by Charles Fritts, who used junctions formed by coating selenium (a semiconductor) with an extremely thin layer of gold (Modern developmentsThe 20th century brought a host of important scientific discoveries and technological advances, including new and better materials and improved methods of fabrication. These developments permitted the enhancement and refinement of many of the energy-conversion devices and systems that had been introduced during the previous century, as exemplified by the remarkable evolution of jet engines and rockets.

They also gave rise to entirely new technologies.